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If you've ever wondered why he's just not that into you, you might be interested in a new genetics study. It explored men's abilities to form strong romantic bonds.

Scientists found that men with a certain form of a gene seemed more likely to have trouble in their relationships.

NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE: The story of this gene starts not with men, but with prairie voles - small furry rodents that have underground burrows. Prairie voles stay with one mate to raise their little ones, and that's intriguing because other species of voles don't do that.

A few years ago, a scientist named Larry Young at Emory University showed the bonding behavior of male prairie voles was linked to a single gene: a gene associated with a brain hormone called vasopressin.

Dr. LARRY YOUNG (Scientist, Emory University): Yeah. We were actually able to take the gene from the prairie vole and inject it into the brain of the meadow vole, which normally should not form bonds. And when we did that, the meadow voles were actually able to form an attachment to their mate.

GREENFIELDBOYCE: Everyone wanted to know: Could this same gene be at work in people? Hasse Walum is a researcher at the Karolinska Institute in Sweden who decided to do a study.

Dr. HASSE WALUM (Researcher, Karolinska Institute): So what we wanted to do was to see if the variation in the same gene was associated with the variation in how humans bond with their partners.

GREENFIELDBOYCE: He and his colleagues had DNA samples from hundreds of men who were participating in a study of twins. The men had also answered questions about their relationships with their spouses or significant others.

Dr. WALUM: Questions like how often do you kiss your partner, and things like that.

GREENFIELDBOYCE: The researchers say that men who had a certain variant of the gene didn't score as high on these measures of bonding. And having two copies of this gene variant doubled the chances that men would report having had a marital crises in the last year. The results are reported in the proceedings of the National Academy of Sciences.

Larry Young, the vole researcher, says other scientists need to confirm the finding to make sure it's real.

Dr. YOUNG: But if it is true, it's very intriguing that the same gene that is involved in pair bonding in a little rodent could be affecting our own relationships.

GREENFIELDBOYCE: But don't expect brides to be demanding premarital gene tests of their grooms anytime soon. Erik Parens is a bioethicist at The Hastings Center in New York. He says the study may show that, as a group, men with this gene variant are more likely to have trouble bonding. But that doesn't mean you can make a prediction for any individual man.

Dr. ERIK PARENS (Bioethicist, The Hastings Center): It's possible to have the gene variant but to not have the marital difficulties. And it's also possible to have the marital difficulties and to not have the gene variant.

GREENFIELDBOYCE: And, Parens says, human relationships are so complicated that the effect of any one gene would be very, very small.

Nell Greenfieldboyce, NPR News.

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